All-solid-state lithium ion secondary batteries contain solid electrolytes and therefore are less flammable than lithium ion secondary batteries containing nonaqueous electrolytes. Proposed examples of the all-solid-state lithium ion secondary batteries include a battery in which a lithium-lanthanum-titanium composite oxide is used as a solid electrolyte is (Patent Document 1) and a battery in which a Li2S—P2S3 composition is used as a solid electrolyte (Patent Document 2). These batteries have not been in practical use. One of reasons for this is an issue relating to solid electrolytes. The solid electrolytes need to have three major properties: high lithium ion conductivity, high chemical stability, and a wide potential window. However, any solid electrolyte having such properties has not been discovered.
Garnet-type oxides have advantages such as high chemical stability and a wide potential window and therefore are candidates for solid electrolytes. However, the garnet-type oxides have disadvantages such as low conductivity. Weppner has recently reported that a garnet-type oxide, Li7La3Zr2O12, synthesized by a solid phase reaction has a conductivity of 1.9×10−4 to 2.3×10−4 Scm−1 at 25° C. and an activation energy of 0.34 eV (Non-patent Document).
Patent Document 1: JP 2008-226639 A
Patent Document 2: JP 2008-084798 A
Non-patent Document: Angew. Chem. Int. Ed., 2007, 46, 7778-7781